Multielectron redox activity facilitated by metal - Metal interactions in early/late heterobimetallics: Co/Zr complexes supported by phosphinoamide ligands

Abstract

To assess the effect of dative M→M interactions on redox properties in early/late heterobimetallic complexes, a series of Co/Zr complexes supported by phosphinoamide ligands have been synthesized and characterized. Treatment of the Zr metalloligands (Ph 2PN iPr) 3ZrCl (1), ( iPr 2PNMes) 3ZrCl (2), and ( iPr 2PN iPr) 3ZrCl (3) with Col 2 leads to reduction from Co II to Co I and isolation of the heterobimetallic complexes ICo(Ph 2PN (Pr) 3ZrCl (4), ICo(′Pr 2PNMes) 3ZrCl (5), and ICo-( iPr 2PN iPr)3ZrCl (6), respectively. Interestingly, treatment of Col 2 with the phosphinoamine Ph 2PNH iPr in the absence of a bound Zr center leads to the disubstituted Co II complex (Ph 2PNH iPr) 2Col 2 (7). The tris(phosphinoamine) Co I complex (Ph 2PNH′Pr) 3Col (8) can only be generated in the presence of an added reductant such as Zn 0, indicating that the reduction of Co II to Co I only occurs in the presence of Zr in the formation of complexes 4-6. Structural characterization of 4-6 reveals a Zr-Co interaction, with interatomic distances of 2.7315(5) Å, 2.6280(5) Å, and 2.6309(5) Å, respectively. This distance appears to decrease as the phosphine donors on Co become more electron-releasing, strengthening the dative Co→Zr interaction. Cyclic voltammetry of 4-6 shows that all three compounds can be further reduced by two electrons at relatively mild reduction potentials (-1.65 V to -2.07 V vs Fc/Fc +). The potentials at which these reductions occur in each of these complexes are largely governed by the extent to which electron-density is donated to Zr, as well as the electron-donating ability of the phosphine substituents. Moreover, cyclic voltammetry of complex 8 reveals that in the absence of Zr, the Co center is significantly more electron rich, and thus more difficult to reduce. Chemical reduction of 5 leads to the isolation of the two-electron reduced dinitrogen complex [N 2Co( iPr 2PNMes) 3ZrX] [Na(THF) 5] (9). X-ray crystallography of 9 reveals that two-electron reduction is accompanied by a significant contraction of the Co-Zr interatomic distance from 2.6280(5) Å to 2.4112(3) Å. These heterobimetallic complexes have been studied computationally using density functional theory to examine the nature of the metal-metal interactions in these species. © 2009 American Chemical Society.